L-Carnitine is present in a living body and functions to carry activated long-chain free fatty acids through the mitochondrial inner membrane. Acyl-CoA derivatives are allowed to enter into the mitochondrial matrix through the mitochondrial inner membrane only in the form of their ester with L-carnitine. Such a carrier function of L-carnitine is exerted in derivering the activated long-chain fatty acids from the site of their biosynthesis. The carnitine found in the living body is exclusively levorotatory, i.e., of L-form, while D-carnitine has never been detected in living bodies.
On the other hand, racemic carnitine has been used for years for various purposes. For example, DL-carnitine has been sold in Europe as an appetizer and also reported to have an effect of promoting physical development of children as described in Borniche et al., Olinioa Chemica Acta, Vol. 5, pp. 171 to 176 (1960). However, in recent years, an importance of using only L-carnitine for therapeutic purposes has been increasing. That is, D-carnitine has turned out to antagonize against carnitine acyltransferase, e.g., carnitine acetyl transferase (CAT) and carnitine palmitoyl transferase (PTC). It has also been elucidated recently that D-carnitine causes depletion of L-carnitine in cardiac tissues. Accordingly, it is essential that cases under treatment for cardiac diseases or for reduction of blood fat should receive only L-carnitine. L-carnitine has thus been recognized as an important medicine.
Known techniques for preparing an optically active carnitine ester and carnitine include (i) optical resolution of a racemate, (ii) biochemical reduction of the corresponding keto ester, and (iii) synthesis using other optically active substances as a starting material or an intermediate.
Examples of the technique (i) include a process in which DL-carnitine amide hydrochloride is subjected to an ion exchange treatment and then resolved using D-camphoric acid as disclosed in JP-A-55-13299 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and a process in which DL-carnitine nitrile is resolved using L-camphor-10-sulfonic acid as disclosed in JP-B-40-3891 (the term "JP-B" as used herein means an "examined published Japanese patent application"). Examples of the technique (ii) include a process in which a .gamma.-halo-acetoacetic ester is converted to an optically active .gamma.-halo-.beta.-hydroxybutyric ester by microbial fermentation and then quaternarized with trimethylamine as disclosed in JP-A-59-118093 and a process in which .gamma.-azidoacetoacetic ester is reduced with a microorganism to obtain an optically active .gamma.-azido-.beta.-hydroxybutyric ester. Examples of the technique (iii) include a process using D-mannitol as disclosed in JP-A-57-165352 and a process using (S)-chloromethyloxirane as disclosed in JP-A-62-212382.
However, the processes (i) utilizing resolution of a racemate as a carnitine precursor require a resolving agent in an amount equimolar to the substrate. Also, the highest possible yield of the desired product, attained if the reaction ideally proceeds, is 50%. Namely, the undesired enantiomer is useless or must be racemized for re-use. The processes (ii) utilizing microbial asymmetric reduction are disadvantageous in that the usable substrate is limited, the production efficiency is low in many cases, and isolation of the product from the reaction solution involves complicated procedures. The processes (iii) using a naturally-occuring optically active substance as a starting material have disadvantages, such as requirement of long reaction steps.